Currencies-related bugs

[MasterPi-2124]

Describe the bug
This is the issue containing all the currency-related bugs during my self-hosted testing (version 1.5.3).

1. In "Exact", I can not fill >3-digit numbers (the community version allowed me to do it)
2. The web is so laggy when handling big numbers. We don't use USD but Vietnamese Dong, which is normal for thousands/millions, unlike USD/EUR. Both behaviors were observed in the self-hosted and community versions
3. Internal server error when enabling simplify debts. (More in reproduce section)
4. Although our currency is VND (I checked the definition files and no digits after the comma), the number is still handled as USD.
5. In Exact section, the numbers below each participant have no meaning. It should reflect the numbers in the input field

To Reproduce
Most bugs described above are related to currency formatting. The below steps apply to the third one.
Steps to reproduce the behavior:

1. Go to 'Groups', click on a group
2. Enable Simplify Debt in group detail (i icon at top-right corner)
3. Switch tab from Expenses to Balances
4. See error

Expected behavior
A clear and concise description of what you expected to happen.

Screenshots
If applicable, add screenshots to help explain your problem.

App version
I used the self-hosted version, v1.5.3

Desktop (please complete the following information):

• OS: [e.g. Mac]
• Browser: Chrome
• Latest Version

[krokosik]

All of these issues should be addressed with #437 PR, which is now released with v1.5.4 along with some other backported fixes. Apologies for the long wait!